1. Field of the Invention
The present invention relates to a magnetic random access memory (MRAM) utilizing a magneto resistive effect.
2. Description of the Related Art
A magnetic random access memory utilizing a tunneling magneto resistive (TMR) effect is characterized in that data is stored in accordance with a magnetizing state of an MTJ (magnetic tunnel junction) element.
The MTJ element showing the TMR has a configuration in which a tunnel insulating layer is placed between two ferromagnetic layers. The MTJ element can take two states. One is a parallel state in which remnant magnetizations of the two ferromagnetic layers sandwiching the tunnel insulating layer are in the same direction, and the other is an antiparallel state in which the remnant magnetizations of the two ferromagnetic layers sandwiching the tunnel insulating layer are in the directions opposite to each other.
When the MTJ element is in the parallel state, the MTJ element has the lowest resistance value. This state is a “1” state. On the other hand, when the MTJ element is in the antiparallel state, the MTJ element has the highest resistance value. This state is a “0” state.
Here, electron spin is quantized in the magnetizing direction, and the magnetizing direction and the direction of the electron spin have a relationship in which they are in the same direction (parallel state) or in the opposite direction (antiparallel state) as described above.
Because energy of electrons differs between the two states, density of states of the electrons in the vicinity of Fermi level of ferromagnetic materials varies depending on whether the magnetizing direction and the direction of the electron spin are the same or opposite. Therefore, a tunnel current flowing through the thin insulating layer sandwiched between the ferromagnetic materials also varies depending on whether the magnetizing direction and the direction of the electron spin are the same or opposite.
In other words, tunneling probability is proportionate to both the density of states of a transition source and the density of states of a transition end, thus when the MTJ element is in the antiparallel state, the density of states of the transition source differs from that of the transition end.
Therefore, the tunneling probability when the MTJ element is in the antiparallel state is increased or decreased as compared with the tunneling probability when the MTJ element is in the parallel state.
It is to be noted that an MR ratio represents a value in which a difference between a resistance value when the MTJ element is in the parallel state and a resistance value when the MTJ element is in the antiparallel state is divided by a resistance value when the MTJ element is in the parallel state or antiparallel state (generally, the resistance value when the MTJ element is in the parallel state is used).
Meanwhile, it is known that the TMR has temperature dependency, but countermeasures against this have not heretofore been adequately taken. Moreover, the TMR has bias dependency, and has a so-called optimum applied voltage that maximizes a signal amount, but this optimum applied voltage and temperature dependency have not heretofore been taken into consideration in designing.